How fast does glass crack? - The Slow Mo Guys

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Gav and Dan use some absolutely insane frame rates to learn how fast 5 millimetre tempered glass cracks. 3 pieces of glass were harmed in the making of this video.
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Filmed at 28,500, 78,000 and 481,000fps on the Phantom V2512
How fast does glass crack? - The Slow Mo Guys

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Hello everyone. I recommend you watch this one full screen or on a computer so you can see the cracks more clearly. They look bloody tiny on a phone screen! - Gav

theslowmoguys

Glass should break at the speed of sound through glass. You can look up the speed of sound in glass and find values between 4 to 5 km/second. So why does the glass seem to break at 1/3rd that speed? Well after thinking it over I realized that just like seismic waves during an earthquake there are different types of waves traveling at different speeds!
So which sound wave are we observing?

The "P" or "primary compression wave" travels fastest at about 4-5km/s in rock like material (glass) and is what is generally used for "speed of sound" but that of course travels far faster than what we see here.

The "S" or "secondary shear wave" has a typical speed around 60% of that of P-waves in any given material. Great! but this means the "S" wave velocity is still too fast and cant be the culprit.

So that leaves the 3rd type of seismic wave: "surface" or "Rayleigh waves". Rayleigh are a lot like the "wave" produced by taking a tight string and flicking it; making the string tighter makes the wave travel faster. Unfortunately this means that "Rayleigh velocity for a sheet of tempered glass" is not something I can easially look up so I will have to approximate using the equation: v=sqrt((N/m^2)/((kg/m^3)/m)

We know that surface of tempered glass has 10, 000 psi (~7000N/cm^2) of compression ( leaving the inside under tension). We also know that the density of glass is 2.5g/cm^3. Plugging this into and simplifying the equation I am left with: sqrt(7000N/.0025kg) = 1700m/s which is close to the velocity observed!

Now this makes sense since the stress that ultimately caused the glass to break was similar to "flicking" the end of a tight rope. I predict that if you hit the edge of the glass with a hammer (or maybe bullet) so that the "p" wave is causing the glass to fail we should observe a much faster (4.5Km/s) glass breaking velocity.

theCodyReeder

So if the earth was made of glass, it would take roundabout four hours till it is completely shattered into pieces.

perschistence

Love the analysis in this video. Dans description of the curvature of the fractures by considering the pane as a section of larger circle is spot on. Not only is it mathematically/physically correct they, explain it in such a way most people can understand it

poundvoucher

2:44 that flash of light was everything

ilian

At 6:00 they debunked the "where's the curve" argument of flat earthers.

fluffyou

Excellent analysis on the failure front.

smartereveryday

To this day it still takes me a minute to remember that Dan is holding the glass in the beginning.

silent

8:08 I think it’s so cool that after all these years they are still mesmerized by what their cameras can do

spaghetti

I don't think shorts were the best clothing option for shattering glass at extremely close range.

bingewatch

The speed measured here is actually the speed of sound in glass!

itisinfactpaul

Dan just accidently explained how the earth is round

drowsiestvirus

You should try to fire a bullet at the same time and then compare the speed of the bullet with the speed of the breaking glass ;)

PressTube

Might be one of the nicest videos, visually, ever made on YouTube. The edits, cuts, little moments... Brilliant video.

Lickwetcarrion

I love that this channel made me actually enjoy learning and it’s just 2 dudes painting rainbows on things and breaking them (for science)

GippyHappy